Electronic devices, in particular integrated electronic devices provide a complex functionality for all kinds of applications. There is a general need to ensure that the electronic device can operate properly in order to avoid malfunctions or failure. There are many different control and monitoring tasks that may be implemented for this purpose. One of the most important issues is to ensure that the power supply voltage level of a power supply for supplying the electronic device is within acceptable limits for proper operation of the electronic device. Monitoring circuits like power on reset (POR) circuitry or other solutions may be employed. There often comparators coupled to the power supply voltage level in order to determine whether or not the power supply voltage level remains within a target window. A major problem of monitoring the supply voltage level are sudden drops or glitches of the supply voltage level with respect to the delay that is usually induced by the comparator until the electronic device may be eventually shut down. It is therefore necessary to guarantee a sufficiently quick reaction of the monitoring circuit in order to minimize reaction time.
The input or output delay of a comparator usually depends on various factors. One of the main factors is the bias current through the differential input pair of a comparator. This bias current, which is also referred to as tail current for differential input pairs, has to be increased in order to increase the speed of the differential input pair. However, generally increasing the tail current entails an increase of the overall power consumption. Therefore, it is useful to increase the bias current only when a quick reaction of the comparator is necessary. This increased bias current should be turned off during normal operation. Some mechanisms that have been developed in order to provide this variation of the bias current are known as “adaptive biasing techniques”. Existing adaptive biasing techniques use, for example, the magnitude of the input voltage difference in order to adapt the tail current of the input stages of amplifiers or comparators through additional amplifier stages. However, this approach also contributes some delay to the automatic adaption process and makes the circuitry much more complex. Furthermore, even the adaptively biased comparators are often too slow, if they have to cope with different supply voltage levels. There are also supply voltage drops or glitches that are so fast that the response time of an adaptively biased comparator is too long.